gdb/testsuite/dap: fix gdb.dap/basic-dap.exp disassembly test for PIE

gdb/testsuite/dap: fix gdb.dap/basic-dap.exp disassembly test for PIE

Prior to this patch, I get:

    >>> {"seq": 17, "type": "request", "command": "disassemble", "arguments": {"memoryReference": "0x115d", "instructionCount": 1}}
    Content-Length: 147

    {"request_seq": 17, "type": "response", "command": "disassemble", "success": false, "message": "Cannot access memory at address 0x115d", "seq": 41}FAIL: gdb.dap/basic-dap.exp: disassemble one instruction success
    FAIL: gdb.dap/basic-dap.exp: instructions in disassemble output

The problem is that the PC to disassemble is taken from the breakpoint
insertion response, which happens before running.  With a PIE
executable, that PC is unrelocated, but the disassembly request happens
after relocation.

I chose to fix this by watching for a breakpoint changed event giving
the new breakpoint address, and recording the address from there.  I
think this is an interesting way to fix it, because it adds a bit of
test coverage, I don't think these events are checked right now.

Other ways to fix it would be:

 - Get the address by doing a breakpoint insertion after the program is
   started, or some other way.
 - Do the disassembly by symbol instead of by address.
 - Do the disassembly before running the program.

Change-Id: I3c396f796ac4c8b22e7dfd2fa1c5467f7a47e84e